The long-term objectives of this application are to better understand the regulation of relaxin secretion in the pregnant rat and to determine the physiological effects of relaxin in both the pregnant rat and pig. The influence of litter size on relaxin secretion during late pregnancy will be determined by adjusting conceptus numbers during early pregnancy and determining relaxin concentrations in frequent blood samplings collected during the antepartum period. The influence of the maternal pituitary on relaxin secretion during the second half of pregnancy will be determined by hypophysectomizing pregnant rats (day 13) which contain either a full complement of conceptuses or one conceptus and determining relaxin concentrations in blood and ovaries collected on days 14, 16, 18, and 20 of pregnancy. The influence of endogenous maternal luteinizing hormone (LH) on relaxin secretion during late pregnancy will be determined by masking LH receptors with the LH antagonist deglycosylated human chorionic gonadotropin and determining relaxin concentrations in the peripheral blood during the last three days of pregnancy. The physiological effects of relaxin at birth in the rat will be determined by observing the consequences of deprivation of endogenous relaxin. This will be done by passive neutralization of relaxin with antibodies for rat relaxin and also by reduction of endogenous relaxin levels by removal of nearly all of the corpora lutea. The influence of relaxin on the tensile properties and biochemical composition of cervical connective tissue during pregnancy in the pig will be determined by comparing these characteristics among cervices obtained from intact pregnant pigs and ovariectomized pregnant pigs treated with progesterone or progesterone plus relaxin. The cervical creep rate of freshly collected sections of the cephalic and caudal portions of the cervix will be determined. The biochemical components of the cervix which will be characterized are those of the glycosaminoglycans, proteoglycans, and collagen.